This invention relates to a method of making a welded can body, more particularly, to a method of making a welded can body having a mash seam weld.
In case of a welded can body which was made by forming a can body preform having a side lapped portion from a blank of a surface treated steel sheet, e.g. tinplate, tinfree steel and a thin-nickel-plated steel or the like, without removing the surface treated film thereof such as a chromium surface layer from the marginal edges forming the side lapped portion, and mash seam welding the lapped portion, it is desirable that the heat affected zone in the weld is continuous in the longitudinal direction thereof, so as to assure sealability in a worked portion such as a flange portion, a necked-in portion and a beaded portion.
In this specification, the heat affected zone is defined as a region where some change occurred in the structure thereof due to thermal hysteresis during welding, presumably by being heated to a temperature of about 1000.degree. C. or above. The boundary surfaces of the lapped portion corresponding to the heat affected zone are united with solid phase weld and/or melt weld to form a firm junction.
When etched with 5% alcoholic solution of picric acid for 2 minutes, a black area 2a, and a gray and/or white area 2b surrounded by the black area 2a are observed in microscopic photographs (refer to FIGS. 1 and 2) of about 50 times magnification. These areas 2a, and 2b constitute the heat affected zone 2 described herein. The black area 2a is supposed to mainly comprise fine precipitates of carbide (including the one in a prodromal stage), and the white area 2b mainly comprise dendrite structure formed by rapid cooling after melting during welding.
A portion of the weld with a heat unaffected zone usually has a poor jointing strength, since iron atoms in the vicinity of the original boundary surfaces have diffused only insufficiently.
Therefore, there has arisen such a trouble that, when a working such as beading, necking, or flanging has been performed on the welded can body, the portions of the weld corresponding to the original boundary surfaces in the heat unaffected zones have peeled off to allow the contents filled in the can to leak or to allow the air to enter the can through the peeled portions, resulting in poor sealability. In case of a tinplate with a normal amount of tin coating (from about 1.0 to 10.2 g/m.sup.2), however, usually the formation of heat unaffected zones would have hardly caused such defects as described above, because a relatively high amount of tin with a low melting point would have supposedly promoted the diffusion of iron atoms.
The continuous heat affected zone as above described may be obtained by mash seam welding the lapped portion of a can body preform by means of an electrode means comprising a cooled (e.g. water cooled) elongated electrode and a cooled rotary electrode having a relatively large radius, preferably of at least 50 mm, preferably having wire electrodes interposed between the lapped portion and the respective electrode, as indicated in U.S. Pat. No. 4,334,138.
However, by only using the aforementioned electrode means, the continuous heat affected zone is not always obtained, and even if obtained, splashes and/or extrusions of molted iron will be caused under inadequate welding conditions, and make it difficult to repair the welds by coating lacquers or the like, resulting in poor corrosion resistance.
Further, it was found that, even if the continuous heat affected zone has been obtained, when conventional sinusoidal alternating current is used for welding, especially when sinusoidal alternating current with high frequency is used for high-speed manufacturing of welded can bodies, a surface pitting 3 due to local melt is produced independent of the heat affected zone 2 in the surface of the mash seam weld 1 on the side of the rotary electrode 5, as shown in FIGS. 1 and 2 (magnification, X50; both etched with 5% alcoholic solution of picric acid for 2 minutes.)
In the figures, reference numbers 4 and 5 designate elongated and rotary electrodes, respectively, and 6 and 7 wire electrodes made of annealed copper wires rolled flat and, as occasion demands, coated with a low melting point metal, e.g., tin. The pitting 3 is mainly produced in the region normally called a shoulder portion corresponding to the neighborhood of the part where the edge face of one side in the lapped portion and the surface of the other side contact with each other.
The pitting 3 not only injures the appearance of a welded can body, but also tends to induce rusting and pinhole corrosion even after the weld 1 has been coated with an organic film, and to cause rupture when the pitting 3 is deep and an area including the pitting 3 has been subjected to beading or the like.